Stall-start methods are used to provide high levels of torque for launching a vehicle. This is useful in achieving the shortest possible 0-60 mph or 0-100 kph times for a predetermined engine configuration. In conventional vehicles having a combustion engine, stall-start methods are initiated by application of the brakes of the vehicle while bringing the engine of the vehicle up to a speed, by pressing the accelerator pedal, where the torque of the engine is increased, in a stall phase. Once the engine is producing high torque, the brakes are released and the vehicle accelerates, in a start phase, under higher torque than would be possible from a standard start condition.
During the stall phase, the engine output rotates while the vehicle wheels remain stationary. A torque converter is often used to permit slip, in a slip condition, between the engine and the wheels. Torque converters have a thermal capacity that is reached in the slip condition after a period of time whose length is dependent on, among other factors, the engine output speed and output torque. Therefore the stall phase can only be maintained until thermal capacity is reached. Torque converters store torque produced by the engine during the stall phase. In the start phase, the torque converter transfers power from the rotating engine output to the wheels as well as the power stored in the torque converter, thus resulting in an amplified torque.
Hybrid vehicles generally feature an electric machine disposed between a combustion engine and the wheels of the vehicle. Commonly, a clutch is positioned between the electric machine and the wheels and takes the place of the torque converter described above. Clutches generally have a lower thermal and torque capacity than torque converters, which are sometimes used in hybrid vehicles but clutches are preferred because of packaging limitations in the hybrid vehicle due to the presence of the electric machine as well as the combustion engine.
The reduced thermal and torque capacity of a clutch compared to that of a torque converter means that the stall phase cannot be held for as long in hybrid vehicles using a clutch as in conventional vehicles using a torque converter, because the clutch would overheat. In turn this means that not as much engine torque can be built up in the stall phase and then transferred to the wheels in the start phase. Furthermore, the clutch does not store torque in the same way as the torque converter such that there is no torque amplification in the start phase.
It is an object of the present invention to at least mitigate one or more of the aforementioned problems.